The PM's decision to withdraw the Tories from the mainstream European People's Party made it inevitable that his party would form eurosceptic partnerships.

David Cameron and Angela Merkel at the EU Council building in Brussels on October 25, 2013. Photograph: Getty Images.

David Cameron fought hard to stop the eurosceptic Germany party Alternative für Deutschland (AfD) from being admitted to the Tory-led European Conservatives and Reformists group. The PM's hopes of a successful EU renegotiation depend on Angela Merkel and the German Chancellor was understandably appalled by the possibility of the Tories teaming up with a right-wing rival to the Christian Democrats (the closest thing Germany has to Ukip).

But in defiance of Cameron's wishes, the group has voted to admit them, with some Conservative MEPs supporting the move. The latest arrival means that the ECR is now the third-largest bloc in the European Parliament, but that will be of no consolation to Cameron. His MEPs have shamlessly defied his authority and further weakened his standing with Merkel (already dented by his "threats" over Jean-Claude Juncker's bid to become EU commission president).

Welcome to the @AfD_Bund party which has joined the ECR Group this morning.

The line from Conservative HQ is that they are "very disappointed" that AfD (which opposes the euro and the US-EU free trade agreement) have been admitted against their wishes and that "the CDU/CSU remains our only sister party in Germany". But while that may be true, Merkel would be within her rights to conclude that she can't do business with a man who can't control his party.

It's a point that Labour has been quick to make, with shadow Europe minister Gareth Thomas commenting:

This shows just how far David Cameron is being pushed around by his own party when it comes to Europe. We know he can’t control his Eurosceptic backbenchers on Europe, and now it seems he’s lost control of his MEPs too.

Just when the Prime Minister needs to maximise British influence in Europe, his MEPs have instead chosen to isolate themselves to the fringes of Europe and alienate our allies.

What started as a political management problem for David Cameron risks turning into a crisis between Britain and one of our most crucial European allies.

David Cameron can’t control his party over Europe, and now it is Britain’s influence and standing in Europe that is at risk of being undermined as a result.

But while Cameron will do all he can to distance himself from the results, the truth is that he only has himself to blame (as Nick Tyrone has previously argued on The Staggers). His decision to withdraw the Conservatives from the mainstream European People's Party in 2009 made it inevitable that his MEPs and others would seek partnership with eurosceptic fringe parties (some, such as the xenophobic Danish People's Party, well to the right of the AfD).

That move was the fulfilment of a pledge made by Cameron during the 2005 Conservative leadership election to appease eurosceptic MPs. But as so often, concessions designed to strengthen his hand have only succeeded in weakening it.